This invention relates to fuel cell production systems and, in particular, to a fuel cell production system with an integrated hydrogen utilization device.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode electrode and a cathode electrode separated by an electrolyte, which serves to conduct electrically charged ions.
As can be appreciated, variable loads, powered by a fuel cell, have placed varying power demands on the fuel cell during its operation. Accordingly, fuel cells must efficiently handle these varying power demands, while producing sufficient power to satisfy the demands. As a result, in order to increase the fuel cell efficiency and to improve the handling of high and low power demands, fuel cell systems have been proposed in which excess hydrogen fuel in the anode exhaust (e.g., in a molten carbonate fuel cell, approximately 10 to 50% of the fuel exits the cell as anode exhaust gas) is stored for future use either by the fuel cell when the fuel demand (power demand) increases or by other devices which use hydrogen as fuel. In addition, also to improve efficiency, fuel cell systems often extract a portion or all of the hydrogen from the anode exhaust, and recycle the extracted hydrogen fuel back to the anode input of the fuel cell.
In one type of arrangement described in U.S. Pat. No. 6,162,556, excess hydrogen that is not consumed during the electrochemical reaction in a high temperature fuel cell is extracted and then collected and stored outside of the fuel cell for future use. More particularly, in the arrangement of the '556 patent, anode exhaust gas containing carbon monoxide, hydrogen, water and carbon dioxide is passed through a shift reactor, where a majority of carbon monoxide is converted together with water into carbon dioxide and hydrogen. The resulting anode exhaust gas is passed through a water extractor and a hydrogen separating apparatus so that essentially only the hydrogen remains in the exhaust. This hydrogen containing exhaust is stored in a storage apparatus and may later be supplied to another hydrogen user.
Other arrangements, described in U.S. Pat. No. 6,320,091 and International Application Publication No. WO 99/46032, employ a storage device for storing excess hydrogen fuel and a means for feeding stored hydrogen fuel to the fuel cell when fuel requirements are greater than the amount of fuel being delivered to the fuel cell. In the case of the '091 patent, a metal hydride system serves as a load leveling device based on gas pressure by storing hydrogen gas delivered from a reformer when the reformer output exceeds fuel cell hydrogen consumption and delivers stored hydrogen to the fuel cell when reformer output is less than the fuel cell consumption. International Application Publication No. WO 99/46032 describes a system employing hydrogen storage means for storing hydrogen produced by a burner module when such fuel is not immediately required by the fuel cell, and a means for feeding stored hydrogen fuel to the fuel cell when the fuel demands are greater than the amount of hydrogen produced by the burner module.
In addition to the above types of systems, other arrangements using multiple fuel cells and fuel cells in combination with other fuel consuming devices, have been used to improve power production as well as handling during high and low power demands. One such system is disclosed in the commonly assigned U.S. Pat. No. 4,917,971, where a high-temperature molten carbonate fuel cell is followed by a low-temperature phosphoric acid fuel cell in a tandem arrangement. In another system described in U.S. Pat. No. 6,655,325, a fuel cell is used with an engine and/or a turbine, such that engine exhaust is passed to the anode of a solid oxide fuel cell for production of electricity, and fuel cell exhaust is recycled back to the engine or directed through a turbine to recover additional energy from the exhaust.
The state of the art systems that combine a fuel cell with another fuel consuming device suffer from a number of disadvantages. For example, many of the conventional systems do not have a black start capability and thus require assistance from a power system, such as a grid, to return to an operating condition after shutting down. In addition, the efficiency of the conventional systems is highly dependent on the fuel composition and fuel utilization rates, because fuel cells, particularly direct carbonate fuel cells, must operate at high utilization rates in order to maintain relatively high efficiency. Moreover, the conventional systems typically have high operating costs, including costs of supplying operating materials, such as fuel, oxidant gas and water, to the system components.
It is an object of the present invention to provide an improved fuel cell power production system having greater fuel efficiency, lower capital and operating costs and reduced emissions.
It is a further object of the present invention to provide a fuel cell power production system employing a high-temperature fuel cell integrated with a hydrogen utilization device, and which is capable of processing high temperature, low pressure anode exhaust gas for efficient use in the hydrogen utilization device.
It is yet another object of the present invention to provide a fuel cell power production system having a black start capability.